As organic light-emitting diodes (OLEDs) are recognized as the next-generation display and lighting technology, OLED related technologies are rapidly developing. Organic-based emitting material is one of the most important core technologies in OLED; therefore, much effort has been devoted to this area. Since triplet emitters could fully utilize all excitons generated in the device, they are the focus in the field of emissive material development.
The idea of using phosphorescent materials in OLED was independently introduced by Baldo et al., Ma, and Che et al. in 1998. The world's first phosphorescent OLED (P-OLED) was fabricated using phosphorescent platinum(II) complex [Pt(OEP)] (OEP=octaethylporphin) (Nature 1998, 395, 151-154) and osmium(II) complex [Os(CN)2(PPh3)(X)] (X=4,4′-diphenyl-2,2′-bipyridine) (Synth. Met. 1998, 94, 245-248) as emitting materials respectively. High efficiency P-OLEDs have continuously been fabricated from new phosphorescent materials and are useful in various mobile electronics devices such as cellphones.
Although the phosphorescent materials that are currently used in the OLED industry have high emissive quantum efficiency (more than 60%), improved phosphorescent materials are needed. As a result, continuous attempts have been made in the development of new materials, especially blue-emitting materials.
In addition, most P-OLEDs suffer from high roll-off. In the cases of platinum containing materials, more than 90% roll-off is observed in 1,000 cd/m2 (Applied Physics Letter 92, 163305 (2008), Chemical Communications 2005, 1408-1410, Chemical Communications 2004, 1484-1485, Applied Physics Letter 91, 063508 (2007), Chemistry A European Journal 2010, 16, 233-247). This problem of high efficiency roll-off is due to triplet-triplet annihilation and/or excimer formation. To solve this problem, previous researchers added bulky groups such as tert-butyl groups in the emissive molecules; nevertheless, the P-OLEDs fabricated from these materials still have more than 50% roll-off (Applied Physics Letter 91, 063508 (2007), Advanced Material 2007, 19, 3599-3603).